If you search on eBAy for 'macbook superdrive' (or combo drive or whatever), you should be able to pull up some listings that include model number for MacBook compatible drives.
They're not too expensive (under $100 I believe.) The problem is that it take a bit of work to replace the drive. It's not horrible (I've done similar work on PowerBooks), just too much work to be worth it for me to upgrade from combo drive to super drive. You can find detailed instructions at ifixit.com.